It is known to strengthen otherwise inadequate load-bearing capacity of soil by formation of short aggregate columns, such as those disclosed in U.S. Pat. No. 5,249,892, the subject matter of which is incorporated in its entirety herein by reference. Generally, short aggregate columns are constructed in situ by individually compacting a series of thin lifts or layers of aggregate within a cavity formed in the soil. When each lift is compacted, vertical compaction forces are transferred through the aggregate vertically and laterally outward to the surrounding soil. The column resulting from a vertical “stack” of lifts, each compacted before the next lift is formed and each including aggregate elements, is characterized by the ability to transfer a relatively large portion of the load outward and laterally into the adjacent, prestressed soil. Short aggregate columns have been recognized in the civil engineering field as revolutionary, partly because they provide for increased load-bearing capacity in soil environments which would otherwise tend to make construction of adequate foundations expensive or unfeasible.
Much effort has been expended towards improving short aggregate column feasibility, reducing their cost, and expanding their field of use and improving their construction. One method for forming a short aggregate column is disclosed in U.S. Pat. No. 6,354,766. The patent discloses lasers mounted on independent devices such as tripods, which become an obstruction to a tamping apparatus during construction operations, and which are used to determine the modulus of the completed pier at the end of the tamping operation at the top of the pier. One drawback of the disclosure is that the lasers do not have the ability to account for movement of a hammer system during tamping. More specifically, as the system tamps the column, the hammer and tamper shaft apply dynamic reciprocating motion to the top of the column. The laser system can measure the position of a stationary object. However, the previously disclosed system cannot be used to measure the performance of each lift of placed aggregate during the column construction process. The present invention provides several unique and novel techniques which overcome the limitations of systems such as those of U.S. Pat. No. 6,354,766, and which include novel methods and the use of a novel quality control apparatus that provide the advantages of reducing the construction cost of short aggregate columns and/or improving their construction.
Since short aggregate columns are desirable, in part, because they are economical, it is desirable to provide for construction techniques which reduce the cost of short aggregate columns compared to known construction techniques, such cost reduction being provided, for example, by monitoring column stiffness data in real time during the column construction process, rather than after the column has been completed. Additionally, it is desirable to provide methods and apparatuses for obtaining stiffness and other data from short aggregate columns during construction in order to verify that each production column built on a particular site meets required design criteria.